The present invention relates to plasma arc torches and, more particularly, to a method of forming an electrode for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein which supports the arc. The element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and their alloys. Some electrodes include a relatively non-emissive separator, which is disposed about the emissive element and acts to prevent the arc from migrating from the emissive element to the copper holder.
A problem associated with torches of the type described above is the short service life of the electrode, particularly when the torch is used with an oxidizing gas, such as oxygen or air. More particularly, the gas tends to rapidly oxidize the copper of the electrode that surrounds the emissive element, and as the copper oxidizes, its work function decreases. As a result, a point is reached at which the oxidized copper surrounding the emissive element begins to support the arc, rather than the element. When this happens, the copper oxide and the supporting copper melt, resulting in early destruction and failure of the electrode.
Many conventional electrodes are assembled by pressing the emissive insert into the metallic holder, or by pressing the emissive insert into a relatively less- or non- emissive sleeve or separator that is then pressed into the metallic holder. The interfaces between the press-fit emissive element, separator, and holder are relatively well defined, and thereby negatively affect the thermal conductivity of the assembled electrode. Specifically, heat travelling through the electrode encounters the interfaces, which act as barriers to heat transfer and thus restrict the heat transfer ability of the electrode. In addition, the well defined interfaces act as stress concentrators that may attract the arc and accelerate the demise of the electrode.
In order to help xe2x80x9csmoothxe2x80x9d the interfaces between the emissive element, separator, and holder, the assignee of the present invention has developed a diffusion bonding technique described in a co-pending application with Ser. No. 09/773,847 (xe2x80x9cthe ""847 applicationxe2x80x9d) entitled xe2x80x9cElectrode Diffusion Bonding.xe2x80x9d In the co-pending ""847 application, a post-assembly heating step is described that creates a diffusion bond between the separator and the metallic holder. The diffusion bond softens or smoothes the interface between the two materials, while increasing the bond strength therebetween. As a result, the electrode has a longer operational life.
While the post-assembly heating step of the co-pending ""847 application represents an improvement in the state of the art, further improvements are desired. In particular, a study of the materials used in an electrode shows that most electrodes employ an emissive element comprising hafnium, zirconium, or the like; a separator comprising silver, gold, nickel, or the like; and a metallic holder comprising copper. While the post-assembly heating step of the co-pending ""847 application improves the bond between the separator and the holder, it is desirable to further improve the bond therebetween.
Furthermore, it is also desirable to improve the bond between the emissive element and separator. And while the post-assembly heating step of the co-pending ""847 application is particularly advantageous for improving the bond between materials such as silver and copper, the relatively high temperature resistance of the emissive element and the separator may cause the bond between the separator and the holder to be destroyed if any heat treatment of the emissive element was attempted. Thus, a problem exists in attempts to form a strong bond between both the emissive element and separator, and between the separator and the metallic holder.
The present invention was developed to improve upon conventional methods of making electrodes. It has been discovered that the difficulties of conventional electrodes, namely increasing the life and performance of electrodes for plasma torches, can be overcome by forming an electrode in a two-stage assembly and heating process, wherein strong bonds are formed between the emissive element and separator, and between the separator and metallic holder. Advantageously, the heating step of each stage is adapted according to the particular materials used in the emissive element, separator, and holder so that the bond strength between the elements of the electrode are maximized.
In particular, a method of fabricating an electrode according to the present invention includes forming an assembly by inserting an emissive element having a relatively low work function in a relatively non-emissive separator. The separator, which is formed of a metallic material having a work function greater than that of the emissive element, has inner and outer surfaces wherein the inner surface of the separator and the outer surface of the emissive element are in surface-to-surface contact. The assembly is then heated such that an intermetallic compound is formed between the separator and the emissive element. In one embodiment, the intermetallic compound is formed after heating the separator and the emissive element to between about 1700xc2x0 F.-1800xc2x0 F. 
According to one embodiment, the assembly is positioned in a cavity defined by a metallic holder after the intermetallic compound has been formed between the separator and the emissive element. In particular, the outer surface of the separator is in surface-to-surface contact with the cavity defined by the metallic holder. After the assembly is in place, a eutectic alloy is formed between the separator and the metallic holder. In one embodiment, the eutectic alloy is formed by heating a copper metallic holder and a silver separator to between about 1400xc2x0 F.-1450xc2x0 F., and more particularly to about 1430xc2x0 F.-1435xc2x0 F., as this is a preferred eutectic forming temperature for these materials. The eutectic alloy forming step is a relatively rapid procedure, wherein the separator and the metallic holder are heated to the eutectic forming temperature for about 0.02-20 minutes. The assembled electrode can also be crimped to provide improve the strength of the electrode.
The intermetallic compound and the eutectic alloy according to the present invention each provide a superior bond between the emissive element and separator, and the separator and metallic holder, respectively. In particular, the intermetallic compound and the eutectic alloy preferably have thicknesses that are greater than that of a diffusion bond, so that the electrode is more strongly bonded together and thus has a longer operational life.
Thus, the present invention provides electrodes and methods of making electrodes having stronger bonds between the elements thereof, which improves the strength and operational life span of the electrode. Furthermore, the methods of making an electrode according to the present invention are directed to electrodes that do not require brazing materials, coatings, or other layers present between the emissive element, separator, or metallic holder. In this regard, the cost and complexity of fabricating the electrode is reduced.